This invention relates to an integrated circuit package having one or more lead-frame leads serving both as a ground lead and heat sink means, and more particularly pertains to such a package in which the one or more leads are an extension of and thus are connected to the die-attach pad and are connected electrically via a wire to a terminal on the top die surface.
In packages of the prior art, especially those at the heart of which is an integrated circuit die including logic and power devices, it is known to provide a terminal on the top surface of the die that is connected electrically via a fine connecting wire (commonly, of gold, aluminum, or copper) to one lead of a lead frame, wherein that lead extends directly from the die support pad of the lead frame. That one lead will be able to conduct heat from the bottom of the die to the printed wire board circuitry to which that lead would normally be attached by soldering and the like, and thus the lead serves the dual purpose of removing heat and also provides an electrical connection to the terminal at the top of the die. A molded resin body encapsulates the die, the proximal end of all of the lead-frame leads, and all of the fine wire connections between die and proximal ends of the leads.
In the package which is the subject of this invention, namely the SOT-89 package, the prior art practice was that the fine wire lead from the above-mentioned die terminal connects the terminal directly to the proximate end of the lead-frame ground lead which extends from the die attach pad. This design of SOT-89 package has been found to be prone to an unacceptable failure rate after repeated temperature cycling typical of the normal use environment. Temperature cycling tests have indicated that the high failure rate is due to occurrence of a wedge bond (heel) failure of the grounding wire that is used for these devices. The failure mode was identified as a rupture or fracture occurring at the heel of the bond located on the lead frame, particularly on the die pad periphery, on or near the ground lead. Experimental observations indicated that the mechanism of the bond fatigue failure is plastic flow and rupture in the heel area induced by cumulative cyclic strain during thermal fluctuations. The identified failure mechanism can briefly be described as follows: i) An excessive reduction in the heel cross-sectional area, accompanied by plastic deformation, is caused by the edge of the bonding tool. ii) The bond knee, which represents the junction between the heel and the wire span, sustains high localized stress by virtue of stress concentration effects. iii) This stress will be intensified by the superimposed molding stress. iv) As the package undergoes temperature changes under temperature cycling conditions the heel is displaced. The displacement has both a horizontal and a vertical component. The horizontal component results from the shear force which is due to thermal coefficient mismatch between the molding compound and the lead frame, while the vertical component results from the molding compound normal stress. v) Due to very low yield strength and high ductility of the wire, the displacement will produce a large amount of plastic strain, i.e., permanent deformation, at the knee for each temperature cycle. This plastic strain will accumulate during the course of the temperature cycling. vi) In addition, during the high-temperature part of the cycle, a significant reduction in the wire yield strength could occur and the wire can behave as a perfectly plastic material which will yield a very large cyclic strain at the knee and the molding compound interface. vii) As the plastic straining continues and the cumulative magnitude of cyclic plastic strain reaches critical value (wire fracture strength), the heel will rupture at the knee and a fatigue crack can initiate, marking the beginning of the bond failure.
These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is an object of this invention to provide an integrated circuit lead-frame package having one or more simple low resistance ground-lead and heat-sink means and having a high tolerance to repeated temperature cycling.